The act sparked ongoing confrontations between vessels near the rig, leading to a number of attacks and collisions. Further inland, the provocation triggered peaceful anti-China protests that erupted into deadly violence in central and southern Vietnam in mid-May.

The two countries have traded diplomatic barbs ever since.

China insists the rig remains inside its sovereign waters and has accused Vietnam of sending ships to disrupt its legitimate operations. Vietnam maintains that Chinese ships have rammed, sunk and fired water cannons at a fleet of civilian and police vessels attempting to protect its maritime sovereignty.

On June 18, China towed the second oil rig near Vietnam’s Hoang Sa (Paracel) Islands, when China's State Councilor Yang Jiechi arrived in Vietnam for a meeting of the Steering Committee on Vietnam-China Bilateral Relations.

Since then Beijing has also deployed two other oil rigs roughly between southern China and the Pratas islands, which are occupied by Taiwan, according to a Reuters report.

"The Chinese are certainly trying to reinforce its legitimacy to operate in the South China Sea by asserting its oil rig deployment," Yun Sun, a China security policy expert with the Washington-based think tank Stimson Center, told Thanh Nien News.

At the press conference Thursday, Binh, the Vietnamese foreign ministry spokesman, said China also started work on a school on Phu Lam Islet -- located in the Hoang Sa Islands.

On June 23, a fleet of Chinese vessels rammed and badly damaged a Vietnamese Fisheries Surveillance ship far away from the Haiyang Shiyou oil rig.

He condemned the action as a serious violation of Vietnam's sovereignty that hindered maritime freedom and safety in the region and violated the spirit of the Declaration on the Conduct of Parties in the South China Sea.

“Vietnam demands that China pay for the damages it caused to the Fisheries Surveillance ship on this occasion and in previous attacks,” he said.

Vietnamese Prime Minister Nguyen Tan Dung has said his government will consider taking legal action against China to resolve the dispute. In March, the Philippines submitted a case to an arbitration tribunal in The Hague, challenging China's claims in the East Sea.